The present invention relates to a semipermeable composite membrane on a microporous substrate having on its surface a semipermeable membrane comprising a crosslinked polymer comprising the reaction product of a mixture of
(a) a water-soluble organic polymer with reactive amino group; PA1 (b) a polyamine and PA1 (c) a crosslinking agent containing a polyfunctional group being able to react with the reactive groups of the water-soluble organic polymer and also with the polyamine, this polyfunctional group having reacted with the amino groups of the organic polymer and with the polyamine.
Semipermeable composite membranes of the abovementioned type, as known from German Offenlegungsschrift No. 2.950.623, have the advantage that the membrane is especially useful for the preparation of pure water from sea water and is eminently resistant to degradation in the presence of chlorine.
The good desalination is evident from the high salt retention of 99% which is obtained if a polyepihalohydrin or a polyethyleneimine is used as the soluble polymer.
This known membrane presents however, the disadvantage that the permeability of the membrane is relatively low as according to example XII of said German Offenlegungsschrift No. 2.950.623 at a retention of 99.6% a flux of 16 l/m.sup.2.h. is obtained.
Because of this low flux membranes of this type are therefore unsuitable in practice.
A similar membrane is known from German Offenlegungsschrift No. 3.013.171 in which use is made of polyalkyleneimines or polyalkyleneimine derivatives as the soluble polymer together with cyclic polyamines and crosslinking agents for the manufacture of the membrane thereby maintaining a ratio of polymer to polyamine of 1:0.05 to 1:1. This membrane does in fact give a high flux of 111 l/m.sup.2.h with a retention of 98.9%.
This known membrane has the disadvantage that a very strictly described combination of measures has to be met so that the manufacture of the membrane requires considerable expertise, as is evident from the comparison of Example 12 in German Offenlegungsschrift No. 2.950.623 with Example 18 in German Patent specification No. 3.013.171. In addition, the polyethyleneamine available commercially is a polymer with branched chains which does not favour constant polymer raw material quality.
Among the water soluble polymers being suitable for use in membranes of this type EP-A-0 031 730 mentions polymers of the formula --CH.sub.2 --CH(NHR.sub.16)-- wherein R.sub.16 is an alkylgroup comprising 1 to 5 carbon atoms.
However, in order to obtain membranes having good properties an additional substance such as e.g. diethyl tartrate or glycol dichlorohydrine has to be added. Said additional compound does not react with the reactive amino groups of the polymer during the interfacial netting but only thereafter by increasing the temperature in order to bind the remaining reactive amino groups.
Apart from the addition of the additional substance and curing at two temperature levels, the polymer can practically not be obtained in a pure state as appears from the fact that this polymer is not used in the practical embodiments.